Electrical power assisted steering

ABSTRACT

An electric power assisted steering system is disclosed in which a motor ( 1 ) is adapted to provide an assistance torque to an output shaft ( 12 ) through a worm gear ( 13 ) and wheel gear assembly ( 11 ) and a biasing means ( 14 ) is provided which biases the shaft carrying the worm gear ( 13 ) into engagement with the shaft carrying the wheel gear ( 11 ). The biasing means ( 14 ) may comprise a spring member provided between the gearbox housing and the bearing ( 9 ) supporting the free end of the worm gear shaft, or a torsion spring adapted to act upon the bearing assembly.

This invention relates to improvement in gear assemblies, and inparticular to electrical power assisted steering assemblies whichincorporate a worm and wheel gear assembly for transferring torque froman electric motor to a steering column or output shaft operativelyconnected thereto.

It is known to provide a power steering system for a vehicle comprisingan electric motor having a stator and a rotor, an input shaftoperatively connected to the rotor and adapted to rotate therewith, anoutput shaft associated with a steering column, and a gearbox adapted totransfer torque from the input shaft to the output shaft in response toa measure of torque in the output shaft produced by a torque sensor. Themotor is typically operated to apply an increasing torque to the outputshaft as the measured torque increases, thus applying an assistancetorque which helps to steer the vehicle.

In a simple arrangement, the input shaft defines a worm gear, and theoutput shaft is provided with a wheel gear which is adapted to mesh withthe worm gear. Whilst such a system is relatively effective, thereexists a problem with noise and vibration due to incorrect meshingbetween the worm and gear wheel. This incorrect meshing may arise due tomanufacturing tolerances, thermal changes in dimensions, distortion dueto torsional loads and wear during service.

We are aware of U.S. Pat. No. 4,967,859 which discloses an electricpower apparatus comprising all the features contained in thepre-characterising clauses of claims 1 and 18. Specifically, theapparatus of U.S. Pat. No. 4,967,858 does not include any form ofbiasing for the shaft.

We are also aware of EP-A-0420131 which discloses a rear wheel steeringapparatus in which a worm shaft extending from a motor is biased towardsa worm wheel by a resilient means in the form of helical spring.

In J-A-62255618 a resilient element constituted by leaf springs has aplurality of drive faces which co-operate with drive faces defined on arotor and on an input shaft.

According to a first aspect, the invention provides an electric powerassisted steering system comprising a housing, an electric motor fixedrelative to the housing having a stator and a rotor, an input shaftoperatively connected to the rotor, an output shaft operativelyconnected to a steering column, and a torque sensor adapted to producean output signal indicative of the torque in the output shaft, the motorbeing adapted to apply a torque to the output shaft dependent upon theoutput signal from the torque sensor through a worm gear provided on theinput shaft which is adapted to mesh with a wheel gear on the outputshaft, the steering system being characterised by further comprising afirst bearing means which supports the input shaft relative to thehousing at its end distal from the motor and a resilient biasing meansadapted to act upon the first bearing means to bias the input shafttowards the wheel gear.

Preferably, the input shaft is biased in a tilting movement which iscentred at a second bearing means which supports the input shaftrelative to the housing at its end adjacent to the motor.

The biasing means may be adapted to apply a sufficient biasing force tothe first bearing means to maintain a fully meshed engagement betweenthe teeth of the worm gear and the teeth of the wheel gear over apredetermined range of torque values carried by the wheel gear. Thishelps to prevent gear rattle when driving straight ahead or on roughroads by ensuring both sides of the engaging teeth on the worm and wheelare in contact at substantially all times over this range of torques.Because the arrangement increases quiescent friction in the gearbox itis important to maintain control of the force applied by the biasingmeans over the full range of the input shaft that is required. Thereforethe biasing means must have a low spring rate.

The provision of the biasing means allows a controlled biasing force tobe applied whilst permitting sufficient tilting movement of the inputshaft to compensate for changes in dimensions due to manufacturingvariations and temperature changes etc. The maximum torque value up towhich the fully meshed engagement is effective is carefully chosen (bycompromise) to avoid excessive friction.

The biasing means may comprise a resilient spring of any type adapted toact between a portion of the housing and the first bearing means.

In some configurations, it is preferred that the resilient springcomprises a leaf spring which may be attached to the housing at a firstend and act upon the first bearing means at its second end. This mayengage the first bearing means at the opposite side of the input shaftto the wheel gear so as to bias the worm into contact with the wheelgear.

The leaf spring may be provided outside of the housing and the secondend of the leaf spring may pass through an opening in the housing toengage with the first bearing means. The second end of the leaf springmay carry a seal which seals with the opening through which it passes.

The input shaft may be directly connected to the motor rotor. It mayextend continuously through the rotor.

The input shaft may be operatively connected to the rotor through aflexible coupling which allows the worm to tilt without movement of therotor.

The flexible coupling may comprise a resilient element, for example ofrubber. The motor rotor may be adapted to apply a drive force to theresilient element through one or more circumferentially spaced radiallyextending surfaces of the resilient element. The resilient element mayin turn be adapted to apply a drive force to the input shaft through oneor more other circumferentially spaced radially extending surfaces ofthe element. The element may comprise a spider shape having a multipleof arms presenting a number of radially extending, circumferentiallyspaced drive surfaces.

Where the input shaft is connected to the motor rotor by a flexiblecoupling, a first compression means may be provided between the housingand the first bearing means at the end of the input shaft distal fromthe motor which applies a compressive force onto the input shaft to biasit towards the motor rotor. It may comprise a coil spring. Its functionis to prevent noise and vibration due to axial free play in the secondbearing means. In its compressed state, there should be free spacebetween adjacent coils in order to avoid frictional resistance to thetilting motion of the input shaft.

A second compression means (such as a coil spring) may also be providedbetween the end of the input shaft adjacent the motor rotor and themotor rotor. This may be provided in a cup formed on an end of the inputshaft whilst a pin extending about the rotational axis of the motorrotor and forming a part of the rotor projects into the cup to engagethe spring.

Thus, whilst the first compression means biases the second bearing meansthrough the flexible coupling, the second compression means biases therotor directly through the pin. There must be a difference between theforces provided respectively by the first and second compression meanswhich is sufficient to bias the second bearing means in the direction ofthe input shaft axis by the desired amount.

In an alternative, the biasing means may comprise an annular O-ringprovided between the first bearing means and the housing. The O-ring maybe of rubber, and may contact an outer circumference of the firstbearing means and a portion of the housing.

The first bearing means may therefore move relative to the housingagainst a resistant force applied by the O-ring as at least part of theO-ring is compressed. In such an arrangement, the wheeled gear or theworm gear may deliberately be made slightly oversize relative to thedimensions which are calculated according to the distance between theirrespective shaft axes. This ensures the O-ring is always under a smallamount of residual compression.

In an alternative to an O-ring, the resilient biasing means may comprisea resilient element which is accommodated between the first bearingmeans and the housing, such as a rubber spacer block. The element may bedisposed between the first bearing means and the housing opposite to theside of the input shaft which engages the gear wheel. The biasing meansmay act in compression or tension.

The second bearing means may comprise a ball bearing which is adapted toprevent radial and axial movement of the input shaft relative to thehousing whilst permitting tilting movement of the input shaft againstthe bias force provided by the biasing means.

The second bearing means may be selected to comprise a high toleranceball bearing assembly which is adapted by virtue of the shape of thegroove in which the balls are located to substantially prevent anyradial displacement of the input shaft relative to the housing as itpasses through the second bearing whilst permitting the input shaft topivot about a point on its axis which passes through a point in theplane of the second bearing means.

In a preferred arrangement, the housing defines a first portion and asecond portion, the first portion comprising a housing for the inputshaft having at least one pair of opposed walls, and having an openingin each wall into which the first and second bearing means arerespectively provided, and the second portion comprising a housing forat least part of the output shaft having at least one pair of opposedwalls, an opening being provided in each wall for receiving one or morebearings adapted to secure the output shaft relative to the housing. Theoutput shaft is preferably mounted orthogonal to the input shaft andsubstantially prevented from moving radially relative to the housing.

A plastic lining portion may be provided around a circumferential outerface of the first bearing means which prevents contact between the firstbearing means and the housing at excessive displacements. This helps toeliminate vibration noise due to metal-metal contact between the firstbearing means and the housing.

The first opening defined in the first portion of the housing (whichreceives the first bearing means) may comprise an elongated slot throughwhich the input shaft passes having semicircular end portions and acentral pair of parallel sides. The spacing between the parallel sidesmay be substantially the same as the outer diameter of the first bearingmeans. The radius of the semicircular end portions of the slot may besubstantially the same as the outer radius of the first bearing means.Thus, the first bearing means may be adapted to move axially along theslot but may be prevented from moving radially perpendicular to theslot.

The first opening of the first portion may comprise an annular borehaving an inner surface with a diameter greater than the diameter of thefirst bearing means. The outer surface of the first bearing means maythus be spaced from the inner wall. The O-ring element forming thebiasing means may be accommodated in this space.

A groove may be provided around a circumference of the inner wall whichlocates the O-ring in a predetermined position relative to the wall, thedepth of the groove being less then the radial thickness of the O-ringelement. In this case, the first bearing means may be displaced from itsrest position against the biasing force provided by the O-ring through adistance equal to the difference between the O-ring radial thickness andthe depth of the groove. Thereafter, the first bearing means isprevented from further radial displacement within the opening as itengages the inner wall of the first opening.

In a yet further alternative arrangement, the biasing means may comprisea torsion bar having a first end acting upon the first bearing means anda second end fixed relative to a portion of the housing so that thetorsion bar applies a biasing force against the first bearing means.

The torsion bar may comprise a substantially U-shaped elongate rodhaving a terminal end portion on a first end of the rod which is bentthrough an angle of approximately ninety degrees relative to theremaining part of the first end and relative to the centre portion ofthe rod to engage with a portion of the first bearing means. Preferably,the terminal end portion acts directly on an outer surface of the firstbearing means opposed to the wheel gear of the output shaft by passingthrough an opening channel in the housing extending radially away fromthe inner wall of the first opening of the first portion of the housing.

The central portion of the torsion bar may be secured to a portion ofthe housing through one or more clamps or shackles.

The second end of the torsion bar may rest upon an end face of athreaded bolt which engages with the housing. Rotation of the boltwithin the threaded bore displaces the second end of the torsion barrelative to the housing. As the first end is engaged with the firstbearing means this acts to increase or decrease torsion in the bar in aknown manner, in turn to alter the biasing force applied to the firstbearing means (i.e. for use when setting up).

In a preferred arrangement, a terminal portion of the second end of thetorsion bar is bent through approximately ninety degrees relative to theremaining part of the second end portion and engages within a recess inthe end face of the bolt. This provides a positive location for thesecond end portion.

In yet a further alternative arrangement, where space in the vehiclepermits, the biasing means may comprise a coil spring having its axissubstantially perpendicular to that of the wormshaft. The coil springcould be installed in a hole in the housing. A first end of the springcould apply force to the outer race of the first bearing means via aformed end of the spring or via a separate component placed between thespring and the first bearing means. A closure plug or plate at the endof the hole distal from the bearing means would provide a support to thecoil spring and a means of sealing.

In a most preferred arrangement, the terminal portion of the second endof the torsion bar engages with a recess in the housing. This rendersthe arrangement non-adjustable and tamper proof which is preferable forproduction versions.

In a refinement, where the biasing means comprises an O-ring seal actingbetween the first bearing means and the housing, the centre axis of theO-ring may be offset relative to the central axis of the input shaft.This provides a different relationship between biasing force anddisplacement of the bearing means compared to the case where the centralaxes of the O-ring and shaft coincide. It is preferred that the axis ofthe O-ring is closer to the wheel gear than that of the input shaftwhere is passes through the first bearing means.

To further refine the relationship between the biasing force anddisplacement of the first bearing means, the shape of the O-ring groove(where provided) relative to the cross-section of the O-ring may bechosen so that the compressed portion of the O-ring just completelyfills the groove at a predetermined displacement corresponding to apredetermined biasing force, whereafter the rate of increase of biasingforce with full displacement is significantly greater than the rate ofincrease of biasing force with displacement at displacements below thepredetermined displacement. When the O-ring is in its normal positioncorresponding to zero torque on the gear wheel, the O-ring may thereforeonly partially fill the groove at this point.

According to a second aspect, the invention provides an electric powerassisted steering system comprising a housing, an electric motor fixedrelative to the housing having a stator and a rotor, and input shaftoperatively connected to the rotor, an output shaft operativelyconnected to a steering column, and a torque sensor adapted to producean output signal indicative of the torque in the output shaft, the motorbeing adapted to apply a torque to the output shaft dependent upon theoutput signal from the torque sensor through a worm gear provided on theinput shaft which is adapted to mesh with a wheel gear on the outputshaft, the steering system being characterised in that the input shaftis operatively connected to the motor rotor by a flexible coupling, andin which the flexible coupling comprises a resilient spider having aplurality of radially extending arms defining a plurality ofsubstantially radially extending drive faces, one or more of the drivefaces co-operating with one or more radial drive faces defined on therotor and one or more of the drive faces co-operating with drive facesdefined on the input shaft.

The input shaft may have a cup formed on its end adjacent the motorrotor. A pin located along the axis of rotation of the rotor may beadapted to be received within the cup. The flexible coupling may beprovided between an end face of the cup and the rotor, perhaps aroundthe pin.

The cup may be adapted to receive a first resilient biasing element suchas a spring which acts between the end of the pin and the base of thecup to bias the rotor away from the input shaft.

A second compression means may be provided which is adapted to bias theinput shaft towards the rotor. This may comprise a spring locatedbetween the housing and a bearing means which supports the input shaft.

There will now be described three examples of the present invention byway of example only. Reference is made to the accompanying drawingswhich include like reference numerals for like parts, of which:

FIG. 1 is a cut-away partial view of a first embodiment of an electricalpower assisted steering system which incorporates the present invention;

FIG. 2 is a view of a second embodiment of an electrical power-assistedsteering system incorporating the present invention;

FIG. 3 is an alternate view of the system of FIG. 2 showing theconnection between the torsion bar and a threaded bolt which is fastenedto the housing;

FIG. 3a is a view of an enlarged portion of FIG. 3;

FIG. 4 is a view of a third embodiment of an electrical power-assistedsteering system incorporating the present invention;

FIG. 5 is a detail of the second support hub for the motor rotor;

FIG. 6 is a detail of the flexible spider element; and

FIG. 7 shows in plan the leaf spring element.

FIG. 1 is a cut-away view of part of an electrical power-assistedsteering system of the present invention for use in a vehicle.

A motor 1 for applying assistance torque to an output shaft 12operatively connected to a steering column shaft comprises a stator 2and a rotor 3. The motor is mounted onto a side of a housing 4. One endof an input shaft 5 which is splined to an end of the rotor extendsthrough an opening 8 into an inner cavity 6 of the housing. The otherend of the input shaft passes through an opening 7 on an opposite sideof the housing to the opening 8, and a first bearing means 9 and secondbearing means 10 located in the openings 7 and opening 8 respectivelysupport the input shaft relative to the housing.

The input shaft 5 carries a worm gear 13 between the two bearing meanswhich is adapted to engage with a toothed wheel 11 provided on theoutput shaft 12 where it passes through the housing. Bearings (notshown) support the output shaft 12 relative to the housing 4 orthogonalto the axis of the input shaft 5 so that the worm gear and wheel gearare meshed.

In use, an output from a torque sensor (not shown) adapted to measurethe torque in the output shaft 12 (or a steering shaft operativelyconnected thereto) is passed to an electronic control unit (ECU) in turnto produce a motor drive signal which controls the torque produced bythe motor 1. The motor 1 then transfers torque through the motor rotor 3to the input shaft 5 and onto the output shaft 12 to provide assistanceto aid a driver of the vehicle.

Each of the bearing means 9, 10 comprises a ball bearing or rollerbearing cartridge having an inner bearing race which co-operates withthe input shaft and an outer bearing race spaced around the inner race,bearings being provided therebetween. Any well known bearing assemblycan be used subject to meeting the requirements for tolerance and loadbearing set out by the designer.

The second bearing means 10 is secured to the housing 4 and acts as apivot about which the input shaft 5 may tilt. It prevents substantiallyall radial movement of the shaft 5 as it passes through the bearing 10.

The first bearing means 9 is spaced from the housing 4 by a resilientbiasing means 14 adapted to bias the input shaft 5 towards the gearwheel 12 of the output shaft 11.

The second (and larger) bearing means 10 therefore reacts tangentialforces being applied to the gear wheel by the worm, as well as radialforces (i.e. at right angles to the axis of the worm) due to the helixangle and pressure angle of the teeth.

The first bearing assembly 9 is constrained axially relative to thehousing 4 (as described hereinafter) but is free to move radiallyagainst the biasing force applied by the biasing means 14.

The biasing means 14 acts to bias the worm into mesh with the gearwheelvia an elastic medium and to allow it to adopt a fully meshed condition(i.e. where there is no clearance between the flanks on either sides ofthe engaging worm and gearwheel teeth) for the range of gearwheel sizeand position variations (due to manufacturing tolerances), temperaturesand states of tooth wear. As shown in FIG. 1, the biasing meanscomprises an O-ring which locates in a groove 15 having a square crosssection.

It is required to maintain this fully meshed condition for a range oftorque values, measured at the gear wheel, (for example up to 4 N-m inone application) in order to prevent gear rattle when driving around thestraight ahead on rough roads. A force of 20 N needs to be applied tothe worm, radially with respect to the gearwheel in order to maintainfull meshing at 4 N-m gearwheel torque. When a higher torque is applied,then the worm will move away from the gearwheel and clearance will occurat the sides of the teeth which are not transmitting the torque. Themaximum torque rating of the gear system shown in FIG. 1 is 42 N-m.

Experiments have shown that the range of dimensional backlash variationdue to tolerances, temperature and wear that may arise, if a biasingmeans was not incorporated, is typically around 0.150 mm. To compensatefor this, a range of radial displacements of the worm, relative to thegearwheel, is needed which is approximately 2× the backlash variation(because of the 14 degree pressure angle); i.e. 0.300 mm total (or+/−0.150 mm from the nominal worm axis position). This range ofdisplacements is provided by allowing the wormshaft to pivot around thelarger ball bearing, nominally moving in a vertical plane, and to biasthe movement towards the gearwheel by means of the 3 mm wide O-ringacting on the outer race of the smaller ball bearing.

In the particular design shown in FIG. 1 the ratio of the lengths fromthe engaging centre of the worm to the centres of the respectivebearings means that a force of 20×48[48+38.5] N (=11 N) should beapplied by the O-ring. This should ideally be maintained over a range ofdisplacements, from the nominal worm axis position, of +/−0.270 mm(=+/−0.150×[48+38.5)/48] mm). However, this is not practical because theforce must change with movement in the case of such a simple elasticmedium. As a compromise, the invention achieves a force range of approx.17 to 27 N over the 0.540 mm (i.e. +/−0.270 mm) total displacementrange. This is achieved by positioning the centre of the O-ring grooveto be offset relative to the nominal position of the worm axis. Theamount of that offset will depend on the Force vs. Displacementcharacteristic of the O-ring, which will act as a non-linear springwhose rate will increase as the worm is forced further away from thegearwheel. Once the limit of the above working displacement range isexceeded, in the direction away from the gearwheel, then the resistanceof the O-ring should rise very rapidly to prevent excessive wormshaftmovement at high torques.

An absolute limit of travel is provided (for example 0.500 mm from thenominal axis) by virtue of the fact the bore in the housing for the 22mm diameter bearing is machined to 23 mm. To avoid metal-to-metal impactnoise, the force of the O-ring at 0.500 mm off centre displacement ofthe small bearing may be chosen to be at least 150 N. This will requirea very fast rising spring rate between 0.300 and 0.500 mm displacement.This can be achieved by tailoring the precise shape of the O-ringgroove, in relation to the diameter of the O-ring's cross section, suchthat the O-ring material just fills the groove at the exact point atwhich the spring rate is required to rise steeply. The hardness of therubber is another parameter that can be optimised.

Note that it is important to limit the meshing force which occurs at thelower torques because it induces a significant amount of quiescentfriction into the operation of the gearbox and this is detrimental formefficiency and good road feel. A meshing force of 20 N will create 0.4 Nof friction as measured at the gearwheel. The maximum acceptable istypically around 0.5 N.

To enable it to act as a pivot centre for the wormshaft, at least overthe small angular displacements involved (+/−0.18 degrees), the largerbearing can be specified as a “C3” clearance grade (i.e. with themaximum standard clearance choice). This will allow the bearing to runwith the required misalignment without excessive friction and wear. Toprevent it from rattling due to the sporadic gearbox torque reversalsthat occur when driving straight ahead on rough roads, it bearing may beaxially pre-loaded at 90N. The pre-load can be applied via the shaft bypreloading the smaller bearing's outer race by means of a compressedcoil spring 16.

The connection between the wormshaft 5 and the motor rotor 3 is viaclearance spline engagement in which a small leaf spring is used tolaterally load the male spline relative to the female spline and removeany torsional free play in the motor drive. This arrangement permits asmall amount of bending compliance between the wormshaft and the motorrotor and hence allows the desired displacement of the wormshaft.

An alternate embodiment is shown in FIGS. 2 and 3 and where possible,like reference numerals have been used to those provided on FIG. 1 asmany components are identical.

The second embodiment differs from the first embodiment in so far as thebiasing means comprises a torsion bar 100 acting upon an outer surfaceof the first bearing means instead of an O-ring seal.

As shown in the embodiment of FIG. 2, a plastic lining portion 17 isprovided around a circumferential outer face of the first bearing means9. The plastic lining portion 17 prevents contact between the firstbearing means 9 and the housing 4.

The torsion bar 100 comprises an elongate bar bent into an elongatedU-shape. Each end of the bar is further bent over though approximatelyninety degrees.

The central portion 101 of the bar 100 is clamped onto a portion of thehousing 4 through a bush which allows the bar to rotate about its axis.One end 102 of the bar acts upon the bearing means whilst the other 103acts upon a bolt 104 threadably engaged with the housing 4. Rotation ofthe bolt 104 displaces the associated end of the bar, inducing torsionin the bar as the other end is prevented from moving by the bearingmeans. Thus, the biasing force can be adjusted by rotating the bolt.

Of course, a bolt is not essential, and many other ways of varying thebiasing force can be employed. For example, shims may be insertedbetween the end of the torsion bar and either the bearing means or thehousing. Alternative spring types are also envisaged within the scope ofthe invention.

The skilled man will therefore understand that the present inventionlies, in at least one aspect, in the provision of a biasing means whichbiases the input shaft towards the output shaft with a desiredDisplacement Biasing Force relationship so as to at least partiallyprevent gear rattle.

FIG. 3a is an enlarged portion of FIG. 3. As shown in FIG. 3a, the firstopening defined in the first portion of the housing 4 may comprise anelongated slot 7 through which the input shaft 5 passes. The elongatedslot 7 has semicircular end portions and parallel sides. The spacingbetween the parallel sides may be substantially the same as the outerdiameter of the first bearing means 9. The radius of the semicircularend portions of the elongated slot 7 may be substantially the same asthe outer radius of the first bearing means 9. Thus, the first bearingmeans 9 may be adapted to move axially along the elongated slot 7 butmay be prevented from moving radially perpendicular to the slot.

A third embodiment of an electrical power assisted steering system ofthe present invention is illustrated in FIG. 4.

This system differs from the previous two systems in that the worm gearis provided on an input shaft 5 which is isolated from the motor rotor 3by a flexible coupling. It is also biased against the wheel gear 11using a leaf spring 200 arranged to act upon the first bearing means 9supporting the end of the shaft 5 opposite the motor.

The motor rotor 3 is cylindrical and is supported at each end by arespective hub 30, 31. A first hub 30 comprises a supporting frame witha bearing located on its axis of rotation. The bearing is a sliding fitover a centred stud 32 which forms a part of the motor housing 4 andwhich is located on the central axis of rotation of the rotor. The studthus passes through the bearing to provide support for the rotor at theend of the motor opposite to the gearbox.

The second hub 31 comprises a radial supporting frame and an integralcentrally located pin 33. The pin 33 projects outwardly from the rotor 3towards the gearbox and is accommodated in a cup 51 formed on the end ofthe input shaft 5 to ensure approximate axial alignment of the rotor andthe input shaft 5.

Between the second hub 31 and the cup 51 is a flexible coupling 52comprising a rubber spider with eight identical, circumferentiallyspaced arms 53 defining sixteen radial drive surfaces 54.

As can be seen in FIG. 6, the second hub 31 has four drive teeth or dogs31 a projecting towards the gearbox which engage between correspondingarms of the flexible spider. The cup 51 on the worm gear shaft issimilarly provided with four teeth or dogs which extend axially towardsthe rotor and engage the remaining drive surfaces between the arms ofthe spider. Thus, drive from the motor rotor is coupled to the wormshaft through the spider.

Providing the flexible coupling, which can be seen in more detail inFIGS. 5 and 7 of the accompanying drawings allows the input shaft 5 tomove without the need for corresponding movement of the motor rotor 3,enhancing the operating life of the motor.

A small coil spring 55 is provided within the cup which acts through aspacer onto the end of the pin of the second hub to bias the motor rotoraway from the gearbox. A second spring 56 is provided between thegearbox housing and the first bearing means to bias the input shaft 5towards the rotor.

The leaf spring 200 can be seen in more detail in FIG. 7 of theaccompanying drawings. It comprises a substantially planar resilientelement that is bolted 202 at one end onto the motor housing with theplane of the spring parallel to the axis of the input shaft.

The free end 201 of the leaf spring 200 is bent through ninety degreesand passes through an orifice in the housing to engage the first bearingmeans 9. This applies a force to the input shaft directed towards thewheel gear. An over moulded seal on the end 201 of the leaf springcooperates with the walls of the orifice to seal the orifice. Inaddition, a cover plate 300 is provided which prevents access to theleaf spring unless the cover is removed.

The first bearing means supporting the free end of the worm gear shaftis located within a plastics guide that is an interference fit withinthe housing. The guide is oversize by 0.76 mm in respect of movement ofthe first bearing means radially towards and away from the wheel gearbut is a close tolerance fit in the orthogonal (horizontal) direction torestrain movement of the bearing in that direction. The guide thusallows only one degree of freedom of movement of the first bearingmeans.

What is claimed is:
 1. An electric power assisted steering systemcomprising a housing, an electric motor fixed relative to said housingand having a stator and a rotor, an input shaft operatively connected tosaid rotor, an output shaft operatively connected to a steering column,and a torque sensor adapted to produce an output signal indicative oftorque in said output shaft, said motor being adapted to apply a torqueto said output shaft dependent upon said output signal from said torquesensor through a worm gear provided on said input shaft which is adaptedto mesh with a wheel gear operatively connected to said output shaft,wherein said steering system further comprises a first bearing meanswhich supports said input shaft relative to said housing at its enddistal from said motor and a leaf spring which is adapted to act uponsaid first bearing means to bias said input shaft towards said wheelgear.
 2. An electric power assisted steering system according to claim 1wherein said input shaft is biased via a tilting movement which iscentered at a second bearing means which supports said input shaftrelative to said housing at its end adjacent to said motor.
 3. Anelectric power assisted steering system according to claim 1 whereinsaid leaf spring is adapted to apply a sufficient biasing force to saidfirst bearing means to maintain a fully meshed engagement between teethof said worm gear and teeth of said wheel gear over a predeterminedrange of torque values carried by said wheel gear.
 4. An electric powerassisted steering system according to claim 1 wherein said leaf springis attached to said housing at a first end and acts upon said firstbearing means at a second end.
 5. An electric power assisted steeringsystem according to claim 4 wherein said leaf spring is provided outsidesaid housing and said second end of said leaf spring passes through anopening in said housing to engage with said first bearing means.
 6. Anelectric power assisted steering system according to claim 1 wherein asecond end of said leaf spring is adapted to carry a seal which sealswith an opening through which it passes.
 7. An electric power assistedsteering system according to claim 1 wherein said input shaft extendscontinuously through said rot or and is sufficiently flexible to allowsaid biasing means to deflect said worm by a required amount.
 8. Anelectric power assisted steering system according to claim 1 whereinsaid input shaft is operatively connected to said rotor through aflexible coupling which allows said worm to move without movement ofsaid rotor.
 9. An electric power assisted steering system according toclaim 8 wherein said flexible coupling comprises a resilient element andsaid motor rotor is adapted to apply a drive force to said resilientelement through at least one circumferentially spaced radially extendingsurfaces of said resilient element.
 10. An electric power assistedsteering system according to claim 9 wherein said resilient element isadapted to apply a drive force to said input shaft through at least onecircumferentially spaced radially extending surfaces of said element.11. An electric power assisted steering system according to claim 8wherein a first compression means is provided between said housing andsaid first bearing means at the end of said input shaft distal from saidmotor which applies a compressive force onto said input shaft to bias ittowards said motor rotor.
 12. An electric power assisted steering systemaccording to claim 11 wherein said first compression means comprises acoil spring.
 13. An electric power assisted steering system according toclaim 8 wherein a second compression means is provided between the endof said input shaft adjacent said motor rotor and said motor rotor. 14.An electric power assisted steering system according to claim 13 whereina cup is formed on an end of said input shaft whilst a pin extendingabout the rotational axis of said motor rotor and forming a part of saidrotor projects into said cup to engage said second compression means.15. An electric power assisted steering system according to claim 2wherein said housing defines a first portion and a second portion, saidfirst portion comprising a housing for said input shaft having at leastone pair of opposed walls, and having an opening in each of said opposedwalls into which said first and second bearing means are respectivelyprovided, and said second portion comprising a housing for at least partof said output shaft having at least one pair of opposed walls, anopening being provided in each of said opposed walls for receiving atleast one bearing adapted to secure said output shaft relative to saidhousing.
 16. An electric power assisted steering system according toclaim 1 wherein a plastic lining portion is provided around acircumferential outer face of said first bearing means which preventscontact between said first bearing means and said housing at excessivedisplacements.
 17. An electric power assisted steering system accordingto claim 1 wherein an opening defined in said first portion of saidhousing which receives a first bearing means comprises an elongated slotthrough which said input shaft passes having semicircular end portionsand a central pair of parallel sides whereby said first bearing means isadapted to move axially along said slot but prevented from movingradially perpendicular to the slot.
 18. An electric power assistedsteering system according to claim 1 wherein said input shafting isoperatively connected to said motor rotor by a flexible coupling, andwherein said flexible coupling comprises a resilient spider having aplurality of radially extending arms defining a plurality ofsubstantially radially extending drive faces, at least one of thecoupling drive faces co-operating with a radial drive face defined onsaid rotor and at least one of said coupling drive faces co-operatingwith a drive face defined on said input shaft.
 19. An electric powerassisted steering system according to claim 8 wherein said input shafthas a cup formed on its end adjacent said motor rotor and a pin locatedalong the axis of rotation of said rotor is adapted to be receivedwithin said cup.